Properties of isotopes: isotopes of the same element differ in which fundamental quantity?

Introduction / Context:
Isotopes are central to nuclear chemistry and applications such as radiometric dating, medical imaging, and reactor fuel cycles. They are nuclei of the same element with identical proton numbers but different neutron numbers, which changes their mass number and sometimes nuclear stability, while leaving electronic behaviour broadly similar in neutral atoms.

Given Data / Assumptions:

• Same element implies same atomic number (same number of protons).
• Neutral atoms when discussing electron configurations.
• Chemical properties may show small isotope effects but remain qualitatively similar.

Concept / Approach:
Mass number A equals protons (Z) plus neutrons (N). Isotopes share Z and differ in N; therefore A differs. This alters nuclear mass and stability but not the fundamental charge that sets electron configuration for neutral atoms.

Step-by-Step Solution:
Note: isotopes ⇒ same Z, different N.Therefore A = Z + N differs.Electronic configuration depends on Z (same for neutral isotopes).Select “Mass number” as the differing quantity.

Verification / Alternative check:
Carbon-12 vs carbon-14 illustrate identical chemical valence with different nuclear mass and stability, confirming that mass number is the differentiator.

Why Other Options Are Wrong:

• Electronic configuration: Same for neutral atoms of the same element.
• Nuclear charge/number of protons: By definition, equal in isotopes.
• All chemical properties: Not entirely different; differences are usually subtle kinetic isotope effects.

Common Pitfalls:
Confusing atomic weight averages with single-isotope properties; assuming large chemical differences where only small kinetic effects exist.

Final Answer:
Mass number